1. Field of the Invention
The present invention relates to a central venous catheter assembly for measuring physiological data for cardiac output determination. Further, the present invention also relates to a method of determining cardiac output.
It is well known and widely applied practice, in particular in the fields of critical and intensive care, to determine cardiac output of a patient from measurement data acquired using catheters and measurement equipment adapted thereto. Therein, various measurement techniques can be applied, which are either based on the principle of observing a dilution process upon inducing a disturbance to the circulatory system (thermodilution, indicator dilution or double dilution techniques) or on the principle of observing blood pressure variation over time and typically evaluating the blood pressure variation on the basis of a modified Windkessel model (pulse contour analysis techniques).
2. The Prior Art
Generally, two catheter application schemes are common. Either a right heart catheter is used, which extends into the pulmonary artery (pulmonary artery catheter), or an arterial catheter (e.g. in the arteria radialis) in combination with a central venous catheter (transpulmonary measurement setup). Increasingly, the latter arrangement is preferred for being less invasive, since application of balloon catheters to the right heart may in some cases lead to severe complications such as malignant arrythmias and pulmonary artery rupture or infarction. However, an even less invasive measurement setup would certainly be welcomed for facilitating clinical practice.
A large variety of invasive cardiac output determination techniques as briefly summarized above are known from a number of publications. In U.S. Pat. No. 5,526,817 a transpulmonary thermodilution setup is described, wherein a cold bolus is injected through a central venous catheter and the temperature response is measured using an arterial catheter. Instead of using cold bolus injections, heated pulmonary artery catheters are used for modified thermodilution methods such as described in U.S. Pat. Nos. 4,507,974 and 5,217,019. Periodic heat pulses at a given pattern are introduced substantially apart from the distal end of the catheter by a heating coil or a thermal filament mounted to the indwelling catheter. Temperature changes of the blood heated when passing the heating coil or thermal filament, respectively, are measured downstream by a thermistor at the distal end of the catheter. Cardiac output is determined quasi-continuously based on the data sampled for several minutes using signal processing and averaging algorithms. Approaches to regulate the heat transferred by the catheter are described in U.S. Pat. Nos. 5,701,908 and 5,857,976. To avoid the above described general risk of pulmonary artery catheters, EP 1 236 435 A1 suggests employing a heated central venous catheter and measuring the temperature response using an arterial catheter equipped with a temperature sensor (transpulmonary setup). EP 1 236 435 A1 further describes combining this technique with pulse contour methods based on arterial pressure readings.
As an alternative to heating blood for thermodilution measurements, it has also been suggested to detect the energy needed for keeping a heat transfer element arranged on a pulmonary arterial catheter at a constant temperature slightly above blood temperature, or, as a similar approach, to measure the resulting temperature of such a heat transfer element when slightly heated with a known supply of energy scheme. Therein, usually two temperature sensors are employed, a first temperature sensor measuring the temperature of the heat transfer element and a second temperature sensor spaced some distance away from the heat transfer element measuring the blood temperature. As higher flow velocities improve heat transfer, the power supplied to the heat transfer element to achieve a given resulting temperature above blood temperature is approximately proportional to flow velocity (depending on Reynolds number). Catheter systems functioning according to this general approach have been described in U.S. Pat. No. 5,509,424 and WO 01/13808 A1. As already mentioned, these systems are based on pulmonary arterial catheters, the application of which is linked with the above-described risks.